Nowadays pedestrian protection in vehicle accidents is stipulated by law, and receives attention as an index of ratings. Because of improvement of the performance of an engine or the like due to an increase in size of the engine, the number of components in the engine compartment has been increasing, thereby reducing a space below a hood, which is necessary for pedestrian protection. Thus, in order to realize both the sporty design and pedestrian head protection, development of a hood structure with which collision energy can be effective absorbed with a small sized space is desired.
The head injury criterion HIC in a pedestrian collision is defined as a maximum product of the 2.5th power of average acceleration applied within an arbitrary time period and an occurrence time period. The HIC is given by the following Math. 1.HIC=[1/(t2−t1)∫t1t2adt]2.5(t2−t1)  [Math. 1]
where a denotes the three-axis resultant acceleration (unit: G) at the center of gravity of the head, t1 and t2 denote points in time having the relationship of 0<t1<t2 to cause a maximum HIC value. An operation time (t2−t1) is specified to be 15 msec or less.
Usually, acceleration G in a collision in which the head of the pedestrian collides with the vehicle is generally classified into a first acceleration peak and a second acceleration peak. The first acceleration peak is generated by a primary collision in which the head collides with the hood panel. After that, the second acceleration peak is generated by a secondary collision in which the hood panel is brought into contact with components disposed in the engine compartment. When a space below the hood is small, the generated collision acceleration is greater in the secondary collision than in the primary collision. Also, a time period during which collision continues is longer in the secondary collision. Thus, the second acceleration peak affects the HIC value more adversely than the first acceleration peak.
Accordingly, in order to reliably achieve the pedestrian protection performance with a limited space, a hood structure is desired, with which a larger amount of energy is reliably absorbed in the primary collision, a crushing load of the hood panel is decreased so as to reliably increase a deformation stroke, and the size of the second acceleration peak is decreased. That is, by decreasing the deformation load of the hood panel, the amount by which the hood panel is deformed is increased, and accordingly, areas not crushed are decreased, and the deformation stroke can be increased.
The hood panel is also required to satisfy the conventionally demanded basic performances such as panel rigidity, dent resistance, flexural rigidity, and torsional rigidity. Panel rigidity is a physical property required for suppressing elastic deformation of the hood panel in the case where a pushing load is applied to the hood panel in such a time when waxing the hood panel or locking the hood panel. The panel rigidity is determined by the Young's modulus of an outer panel and positions where the outer panel and an inner panel are bonded to each other. Dent resistance is a physical property required for suppressing plastic deformation remaining in the hood panel when pebbles or the like fly and hit the hood panel. Dent resistance is determined by proof stress and the thickness of the outer panel. Flexural rigidity is a physical property required for suppressing elastic deformation of an outer peripheral portion of the hood panel occurring due to a pulling force in the vehicle direction produced when the hood is locked and reactive forces produced by cushion rubber, damper stays, sealing rubber, and the like. Flexural rigidity is determined by the Young's modulus (modulus of longitudinal elasticity) and a second moment of area calculated from the shapes of the inner panel and a reinforcement in the outer peripheral portion of the hood panel. Torsional rigidity is determined by the flexural rigidity of the hood panel in the outer peripheral portion and the thickness and the shape of the inner panel in a central portion of a hood.
Patent Literature 1 proposes a technology in which, with a plurality of reinforcement beads provided so as to be parallel to one another in an inner panel, flexural rigidity of the inner panel is improved, bending of the inner panel occurring when the head of the pedestrian collides with a panel is suppressed, and an impact load is widely distributed over the panel. That is, with the reinforcement beads provided in the inner panel, when the head of the pedestrian collides with the panel, the area by which the panel is displaced is increased, thereby increasing the amount of the impact load to be absorbed. Thus, in comparison with the case where the reinforcement beads are not provided, the size of the first acceleration peak can be increased when the head collides with the panel. The reinforcement beads, which are each formed to have a hat shape, are easily flattened when the reinforcement beads are brought into contact with contained components. This can also decrease the size of the second acceleration peak. In Patent Literature 2, the inventors of the present application also proposed a hood panel in which a plurality of reinforcement beads are provided so as to be parallel to one another from the above-described viewpoint.
Patent Literatures 3 to 6 disclose inner panels in which hat-shaped reinforcement heads are formed so as to extend in the vehicle front-rear direction. In these inner panels, the upright walls of the reinforcement beads have a step shape (Patent Literature 3), the depth of the bottom surfaces of the reinforcement beads is varied (Patent Literature 4), or cuts are formed between the reinforcement beads (Patent Literature 5). Furthermore, a technology is disclosed in which the shape of the hat-shaped reinforcement beads provided in the inner panel is varied in the vehicle front-rear direction in view of the fact that, when the pedestrian who collides with the vehicle is a child, the pedestrian tends to collide with a front side of the hood panel, and when the pedestrian who collides with the vehicle is an adult, the pedestrian tends to collide with a rear side of the hood panel (Patent Literature 6).